The present invention relates generally to a pressure relief device as may be used with an electrical apparatus such as a transformer housing. More particularly, the present invention relates to a pressure relief valve for closing a vent opening in a power transformer housing. The pressure relief valve includes gasket means for effecting seals in planes that extend both parallel and normal to the axis of the vent opening and that maintains one seal after the other is broken.
Pressure relief devices are commonly used with electrical apparatus and particularly with high voltage transformers. Such transformers are usually in sealed housings that often contain a dielectric fluid. If, for example, the pressure within the transformer housing should increase due to an increase in ambient temperature, an internal fault or other reason, it may be necessary to vent the pressure in order to prevent a catastrophic failure of the transformer housing. These pressure relief devices are designed so that once a predetermined pressure is reached, a considerable amount of fluids such as hot oil and gas, are vented from the transformer housing in a very short time to rapidly reduce the pressure within the housing. After venting, the pressure relief device closes to again seal the vent opening.
A typical pressure relief device as shown in U. S. Pat. No. 4,676,266 includes a tubular base that is fixed about an opening in the transformer housing. A spring loaded valve disk having a depending skirt is urged down over the tubular base to close the opening. The design of the device shown in the ""266 Patent requires two gaskets between the valve disk and the tubular base. One is a top gasket that lies in a seat formed on the end face of the tubular base. This gasket seals against an under surface of the valve disk. A second gasket is a side gasket, sometimes in the form of an O-ring or a flat annular gasket that is seated in a groove extending about the periphery of the tubular base. This side gasket seals against the inner periphery of the depending skirt and maintains a seal after the seal at the top gasket is broken.
A problem associated with the dual gasket arrangement as disclosed in the ""266 Patent, concerns the retention of the two gaskets in their respective seats during a venting event. Venting is a catastrophic event and the volume and flow rate of the fluids passing over both gaskets during such an event may be so great as to pull the gaskets from their respective seats. In order to prevent the top gasket from being driven from its seat by the venting fluid, it typically is locked in place with a mechanical locking ring. However, there has been no economic mechanical way to secure the side gasket so it has either been left unsecured or an epoxy was used to fix the gasket in its seat. Neither solution is entirely satisfactory.
Accordingly, it is an object of the present invention to provide a pressure relief device having an improved gasket arrangement.
Another object of the present invention is to provide a pressure relief device having an improved gasket arrangement that is less susceptible to a dislocation of the gasket by the force of fluids venting from the housing and through pressure relief device.
Yet another object of the invention is to provide a pressure relief device having a single gasket that effects seals in two planes and that maintains the seal in one plane intact after the other is broken.
A further object of the invention is to provide a pressure relief device in which the functions performed by the dual gasket arrangement of the prior art are performed by a single gasket structure that is mechanically secured in place.
The present invention provides a pressure relief device having a gasket structure that combines features and functions of the top and side gaskets of the prior art in a single structure. The gasket structure provides both a top seal portion and a side seal portion and is arranged to maintain a side seal until after the top seal is broken. Moreover, a pressure relief device having an integral gasket structure is easier to manufacture and maintain since a single mechanical attachment means will prevent dislocation of both gasket portions by the force of the venting fluids.
Accordingly, the present invention may be characterized in one aspect thereof by a pressure relief device for mounting to a housing for hermetically isolating electrical apparatus, the device comprising:
a) a collar attachable to a surface of the housing and about a vent opening in the surface, the collar upstanding from the surface;
b) a valve disk fitting over the collar, the valve disk having a depending skirt surrounding the collar;
c) bias means urging the valve disk axially towards the collar for closing the vent opening;
d) a single gasket having a first generally planar surface for effecting a first seal between the valve disk and the collar in a plane generally parallel to the plane of the vent opening and a second surface extending around the periphery of the collar for effecting a second seal between the collar and skirt in a plane generally normal to the plane of the vent opening, the second surface being arranged to maintain a sealing contact with the skirt after the breaking of the first seal; and
e) a retaining ring for attaching the gasket to the collar.